Hmmmmm.... what photos are you looking at?
I suspect that your choice to "balance" your rig so it wants to pivot forward (increases the load, but not as mast foot pressure) and "load up" may be part of your issue.
If you have alot of pull on your back hand, guess where that force "hits the board".
Yep, you got it, the additional pressure on your back hand (to keep the rig sheeted in and from pivoting forward over the mast foot) goes straight down to your back foot, then straight into the fin.
I do not think that having a bit of bend in your front leg, and a fairly straight back leg is necessarily a bad thing, but putting too much (more than is required) pressure on the fin with your back leg and spinning out the fin is definitely not good.
I'd suggest your move your front line attachments to truly balance your rig, then move the back attachment point back slightly (and perhaps get both closer together to about
one hand width total in between) to take a little pressure off your back hand and back leg.
If you are looking at the pro's, what discipline are they sailing.... slalom, formula, freestyle, waves.....?
Some of it depends on the type of board, the sail size range, mast foot position etc.
Also, 34 cm to 44 cm on a 114 liter board sounds a little extreme to me.
Where do you run your mast foot....and why?